smoothness provides a classification scheme regarding the physical usability of a way for wheeled vehicles, particularly regarding surface regularity/flatness. Usability by a particular type of vehicle means that the vehicle can traverse the route without significant risk of damage (e.g., to vehicle undercarriage/drivetrain) or injury (e.g., by falling). The tag can be applied to all driveable ways and areas: highway=*, parking areas, beaches, etc.

tracktype=* is also an important surface property and refers to surface firmness, regardless of shape.

surface=* refers to surface material and sometimes to material structure. It may suggest a surface smoothness sometimes, but because the same surface material/structure can exhibit various shapes, it is still advisable to add smoothness=*.

Please note: Photos are examples ONLY ! Because the smoothness of a way should be assessed solely based on whether the way is usable by the vehicles mentioned above. This is my whole point: as a user of a certain road or path I am only interested whether I can drive on this road or not.

This table is a wiki template with a default description in English. Editable here.

Usage

In developing countries, the HDM model is using some values of smoothness=* to help define road quality.

Controversy

It is clear from discussion on the mailing lists that a major part of the community does not like this key, based on the guideline of Verifiability.
Please have a look at the discussion page and the mailing-list (thread starting at [1], contiued at [2]) and make up your own mind.
As always on OSM you are free to use the keys you like in the way you like!
Use the page Talk:Proposed features/Smoothness for the controversy details.
Please help improving the tagging of roads by posting constructive comments to Proposed_features/surface_unification.

The key could be made more verifiable changing the values to thin_rollers, thin_wheels, wheels, robust_wheels, high_clearance, off_road_wheels and specialized_off_road_wheels. (See Proposed_features/usability) for a transition.)